Alex Smith's post-concussion symptoms worsened late last week, which opened the door for backup Colin Kaepernick to start Monday's 32-7 win over the Bears at Candlestick Park.

Smith shared the practice reps in the middle of the week, but Kaepernick took all the snaps during a Saturday practice and a walk-through Sunday. The players learned Kaepernick would start Sunday night at the team hotel.

After Monday's game, Smith said he was still feeling the effects of the concussion he sustained Nov. 11 in a 24-24 tie with the Rams. Smith has said his immediate post-concussion symptoms included blurred vision, headaches and nausea.

"I feel better," Smith said. "I'm kind of in the middle of it."

Smith was ruled out after being evaluated by Dr. Dan Garza, the team's medical director and concussion expert.

"It was up to the doctor," Smith said when asked what prevented him from playing. "That's a better question for the doctor."

Young on QBs, concussions: ESPN's Steve Young knows a little bit about the two big story lines entering the game: quarterbacking and concussions. And the former 49ers quarterback knows how much progress has been made when the two topics intersect as they have this week with both starters, Smith and Jay Cutler, ruled out because of concussions.

"The truth is, 15 years ago, neither of these quarterbacks would sit out," Young said on the field at Candlestick Park before the game. "Big games, people on the cusp of feeling better, you'd usually err on the side of playing. I'd be the first one to run out there."

Young often was the first one to run out there. A concussion on a "Monday Night Football" game in Arizona in 1999 ended his career.

On another subject - Kaepernick - it's a good thing the second-year quarterback didn't heed Young's advice. Young said Kaepernick, who completed 16 of 23 passes for 243 yards and two touchdowns, should rely more on his legs than his arm. Kaepernick had four carries for 12 yards.

"Don't throw anything you're not sure of," Young said. "It's not the night to try anything. You cannot take this away from your teammates. You can't be the reason they don't have a chance to win. You can't be the reason they lose."

Briefly: The 49ers have won eight straight against the Bears at Candlestick dating back to 1987. They have outscored them 271-49 during the streak. ... The 49ers got their 43rd "Monday Night Football" win, tying the Cowboys for the most in the NFL. ... Aldon Smith's 29 career sacks are the third-highest total in NFL history for a second-year player (Reggie White 31; Derrick Thomas 30). Smith still has six regular-season games left in his second season. ... Running back Brandon Jacobs was inactive.